Supposing we did get a 20.000 stadium, and then were relegated - of course, there would be less spontaneous demand, that's a given.
But as the advertising/marketing industry shows us every day, there are hundreds of ways of STIMULATING demand. The club doesn't have to sit there passively and hope that a few more people show up for the next game. They's have to get off their arses and do something creative to get people in, something they've forgotten over the past 5 years.
For example, for the less attractive fixtures, we could offer :
Kids For A Quid
Family For A Tenner
Free Scarf & Hat for every kid
Buy One, Get One Free (Bring A Mate, Double The Gate, remember that ?)
Then you could introduce a "frequent flier" type scheme, e.g. for every 5 games you attend, you get some kind of premium, and so on, and so on.
For me, the key thing is to grab the younger generation before they slip away. It's just a hunch, but I sense that if they haven't got the bug by the time they are, say, 12-14, they probably never will.